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Reaction And Synthesis: Greener And Cheaper Ammonia Beckons

Jan. 15, 2020
High-activity oxygen-substituted perovskite catalyst requires relatively mild conditions


Scientists at the Materials Research Center for Element Strategy, Tokyo Institute of Technology (Tokyo Tech), Tokyo, claim a breakthrough in the manufacturing of a catalyst that could be key to greener and lower cost production of ammonia.

Led by professor Masaaki Kitano, the scientists have focused on perovskite, a calcium titanium oxide mineral composed of calcium titanate (CaTiO3) that also is an important material in photovoltaic solar cells. Its name also applies to the class of compounds having the same type of crystal structure as CaTiO3, known as the perovskite structure.

Perovskites are at the heart of many different chemical-related studies because tweaking their composition also changes their properties. One such application is in the production of ammonia.

Ammonia’s synthesis rate is generally limited by the high energy required to dissociate the nitrogen molecules in the traditional Haber-Bosch process.

Replacing some of the oxygen atoms in perovskites with hydrogen and nitrogen ions already has proved an efficient catalyst for ammonia synthesis. The drawback to these chemical substitutions is that synthesis usually requires temperatures exceeding 800°C over a period of weeks.

Kitano and his co-workers devised a novel method for low-temperature synthesis of one such oxygen-substituted perovskite — BaCeO3−xNyHz — and then test its ability to act as a catalyst in ammonia synthesis.

To do this, they altered the perovskite synthesis process. It normally uses barium carbonate and cerium dioxide as precursors and requires very high temperatures to have them combine into the base perovskite BaCeO3 because barium carbonate is such a stable compound. Then there still is the need to substitute the oxygen atoms in the compound with nitrogen and hydrogen ions.

Instead of barium carbonate, the team turned to barium amide and found that it reacts easily with cerium dioxide under ammonia gas flow to directly form BaCeO3−xNyHz at low temperatures and in less time.

The catalysts show very high activity in the range of 240–400°C at 0.9 MPa, much milder conditions than the Haber-Bosch process, which runs at around 500°C and 20 MPa.

The researchers took a bottom-up approach to make the perovskite, flowing ammonia over the barium amide and cerium dioxide at 300–600°C for about six hours.

“This is the first demonstration of a bottom-up synthesis of such a material, referred to as perovskite-type oxynitride-hydride,” says Kitano.

Finally, the researchers attempted to elucidate the mechanisms behind the improved synthesis rate for ammonia. This overall insight will, they hope, both serve as a protocol for the synthesis of other types of materials with nitrogen and hydrogen ion substitutions and for the intelligent design of catalysts.

“Our results will pave the way in new catalyst design strategies for low-temperature ammonia synthesis. These findings will hopefully make the synthesis of useful materials cleaner and more energy efficient,” he adds.

The breakthrough is the latest development in the scientists’ multi-focused quest to improve the efficiency of ammonia synthesis. Kitano also is involved in a research group at Tokyo Tech that investigated the efficacy of nanoparticles of ruthenium immobilized onto a calcium amide catalyst doped with barium.

The results showed catalytic activity 100 times greater than that of conventional ruthenium catalysts at temperatures below 300°C. Further, the performance of this catalyst is over three times higher when compared to iron catalysts being used industrially.

A thin barium layer is formed on ruthenium nanoparticles about 3 nm in size during the reaction of this catalyst, as electrons with low work function and porous calcium amide form concurrently due to amide deficiency. Both these properties indicate high catalytic activity, note the scientists, who discovered that this catalyst operates with the active structures forming in a self-organized way and remaining stable throughout the reaction.

The Japanese researchers say the catalyst developed in this research far supersedes the limits of existing catalytic materials in its ammonia synthesis activity and could contribute significantly to reducing energy used for the ammonia synthesis process. As a result, further development of this technology could lead to a new process structure for on-site synthesis of ammonia, they believe.

Seán Ottewell is Chemical Processing's Editor at Large. You can email him at [email protected].

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